Numerous techniques have been employed for forming thin-walled work pieces, including in particular, longitudinal welding and drawing/redrawing/ironing techniques used in forming three- and two-piece cylindrical metal container bodies, respectively. Subsequent modifications to metal container bodies can be achieved via die necking, roll or spin necking, and other secondary processes.
Die necking generally entails forcing the sidewall of a container body and an external die against one another, typically by relative longitudinal advancement of the container body through a concentric outer die. In roll and spin necking the sidewall of a container body is contacted by an external roller, and in some instances an internal roller, that can be contoured and/or radially/axially advanced to neck the container body. Recently, symmetric longitudinal flutes or ribs, and diamond, waffle and numerous other patterns have been imparted to cylindrical container bodies through the use of either an internal roller and an external compliant mat, or by an internal roller and a matching external rigid forming element. Expanding manderals have also been utilized on three-piece metal container bodies to impart such patterns.
The noted techniques are limited as to the diametric extent and complexity of shaping that can be achieved. By way of example, die-necking cannot readily be employed for current aluminum drawn and ironed beverage containers (e.g., containers having a sidewall thickness of about 4-7 mil.) to achieve diametric changes of more than about 3% in any single operation and does not generally allow for container diameters to be increased then decreased (or vice-versa) or for discontinuous/angled designs to be shaped along the longitudinal extent of a container body. While spin forming techniques have been found to allow for relatively high degrees of expansion (e.g., in excess of 15% for current aluminum drawn and ironed beverage containers), relative rotation between a container body and the forming roller is necessary, thereby restricting the ability to achieve non-circular cross-sections along the longitudinal extent of a container body.
Other proposed techniques also have limitations. For example, electromagnetic and hydrostatic processes have been considered which entail the use of magnetic fields and pressurized vessels, respectively, to force a container body sidewall outward against an outer shaping die. Both processes require, however, a container body to be of sufficient ductility to withstand substantial attendant plastic deformation without failure. For current drawn and ironed aluminum beverage containers, such deformation limits are believed to be less than 5% before failure is realized due to the limited ductility of the aluminum alloys utilized. While annealing such container bodies may provide sufficient ductility to allow a greater degree of metal deformation, it would lower the strength of the container bodies and require additional undesirable thermal processing.